High trophic niche overlap in mixed-species colonies using artificial nests

Although successful at recovering endangered populations, conservation actions based on nest provisioning seldom consider how they shape the composition of communities and alter interspecific interactions. Specifically, the extent to which dietary overlap within these communities may affect the conservation of target species has rarely been assessed. In Southern Europe, sites of large-scale nest-site provisioning aimed at recovering Lesser Kestrels Falco naumanni populations have attracted non-target bird species, resulting in mixed breeding assemblages that might promote interspecific competition for resources during breeding. Here we used stable isotope analysis (δ ¹⁵N and δ ¹³C) to assess inter- and intraspecific dietary segregation in these assemblages and investigate the mechanisms allowing species coexistence. We examined resource partitioning and trophic niche overlap among Lesser Kestrels, Common Kestrels Falco tinnunculus, European Rollers Coracias garrulus, Western Barn Owls Tyto alba, Little Owls Athene noctua and Spotless Starlings Sturnus unicolor; and within species between parents and their offspring. Similar isotope ratios and highly overlapped niches, particularly among Lesser Kestrels, Rollers and Starlings, suggest limited dietary segregation and use of similar prey. Within species, parent–offspring segregation was marked across all species. Our results indicate that species breeding in these assemblages occupy similar ecological niches, despite a potential increase in competition. High resource availability in the area may permit coexistence but the viability of mixed-species groups may be compromised in areas with limited resources, which are predicted to expand with ongoing human- and climate-induced changes. Conservation practices based on nest provisioning should consider the ecological niches of target and non-target species as well as their interactions.